Data from: Specificity in the symbiotic association between fungus-growing ants and protective Pseudonocardia bacteria

Fungus-growing ants (tribe Attini) engage in a mutualism with a fungus that serves as the ants' primary food source, but successful fungus cultivation is threatened by microfungal parasites (genus Escovopsis). Actinobacteria (genus Pseudonocardia) associate with most of the phylogenetic diversity of fungus-growing ants; are typically maintained on the cuticle of workers; and infection experiments, bioassay challenges and chemical analyses support a role of Pseudonocardia in defence against Escovopsis through antibiotic production. Here we generate a two-gene phylogeny for Pseudonocardia associated with 124 fungus-growing ant colonies, evaluate patterns of ant–Pseudonocardia specificity and test Pseudonocardia antibiotic activity towards Escovopsis. We show that Pseudonocardia associated with fungus-growing ants are not monophyletic: the ants have acquired free-living strains over the evolutionary history of the association. Nevertheless, our analysis reveals a significant pattern of specificity between clades of Pseudonocardia and groups of related fungus-growing ants. Furthermore, antibiotic assays suggest that despite Escovopsis being generally susceptible to inhibition by diverse Actinobacteria, the ant-derived Pseudonocardia inhibit Escovopsis more strongly than they inhibit other fungi, and are better at inhibiting this pathogen than most environmental Pseudonocardia strains tested. Our findings support a model that many fungus-growing ants maintain specialized Pseudonocardia symbionts that help with garden defence.

植菌蚂蚁（fungus-growing ants，菌蚁族Attini）与一种真菌形成互利共生关系，该真菌是蚂蚁的主要食物来源，但成功的真菌栽培会受到微真菌寄生虫——埃斯科沃氏霉属（Escovopsis）的威胁。放线菌（Actinobacteria）与绝大多数植菌蚂蚁的系统发育多样性相关联；它们通常定殖于工蚁的体壁；感染实验、生物测定攻毒试验以及化学分析均表明，假诺卡氏菌属（Pseudonocardia）可通过产生抗生素发挥抵御埃斯科沃氏霉的作用。在此，我们针对124个植菌蚂蚁群落所关联的假诺卡氏菌属菌株构建了双基因系统发育树，评估了蚂蚁与假诺卡氏菌的特异性模式，并测试了假诺卡氏菌对埃斯科沃氏霉的抗菌活性。我们发现，与植菌蚂蚁关联的假诺卡氏菌并非单系群：在该共生关系的演化历程中，蚂蚁曾多次获得自由生活的菌株。尽管如此，我们的分析揭示了假诺卡氏菌演化支与相关植菌蚂蚁类群之间存在显著的特异性模式。此外，抗菌试验结果表明，尽管埃斯科沃氏霉普遍对多种放线菌的抑制作用敏感，但源自蚂蚁的假诺卡氏菌对埃斯科沃氏霉的抑制效果强于其对其他真菌的抑制效果，且相较于绝大多数受试环境假诺卡氏菌菌株，它们对该病原菌的抑制能力更强。我们的研究结果支持这一模型：多数植菌蚂蚁会维持特化的假诺卡氏菌共生体，以帮助其守护真菌花园。